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Aviation History
1953
1953 - 0993.PDF
31 July 1953 147 Concerning the drag problem, one had to consider how the mass flow of air into the engine intake was affected by, and itself affected the normal flow over the aircraft as a whole. If the engines were situated well away from the centre line of the aircraft, one might accept the fact that the flow at these positions would tend to be tha due to the presence of a normal aerofoil section. At certain speeds, the engine intakes would inhale more air than would be delivered throueh its frontal area due solely to the passage of the free stream. At higher speeds, there would be spillage around the outside, interfering with the normal flow pattern. There was clearly an optimum speed at which the intakes were operating most efficiently, and this speed could be varied by altering the intake area. It would seem, therefore, that the effect of the engine position upon drag could be adequately met by varying the intake area, having regard to the local air velocities From the flutter pomt of view, it could appear that, paying due attention to the possible energy extraction from the wings because of their chord lengths, the spanwise position of the nodal lines of the aircraft's fundamental mode of vibration needed to be somewhere in the neighbourhood of 50 per cent semi-span. Since these nodal lines were likely to occur near the concentrated masses of the engines, the latter should themselves be at about 50 per cent semi-span. With regard to the accessibility of the engines, it was obvious that the maximum serviceability of the aircraft could be maintained only if speedy and simple removal of the engines was possible for maintenance fig. 3. The configuration suggested by Mr. Hunn's discussion on the development of a design. Most important requirement to be met in evolving any such design is that of a low drogjthrust ratio. purposes. On current aircraft, it was frequently found convenient to construct the wings in two sections, the outer wing and the stub wing which was integral with the fuselage structure. It was suggested that this junction of the two wing sections was a convenient position to place the engines. This might not be at 50 per cent semi-span on most current aircraft, but in the design which would subsequently evolve this would be the case. The mass and air loading needed to be made as compatible as possible in order to minimize aeroelastic distortion in flight. It could reasonably be assumed that, to a first approximation, the spanwise lift distribution on the wings was elliptic. Small modifications of this pattern might be effected, but this assumption would suffice as an adequate starting point. For the aircraft to possess a useful high-speed duration, large fuel capacity was needed. This quantity of fuel might easily exceed in weight that of the engine combination, and it therefore followed that throughout any sustained flight, considerable changes in mass distribu tion must occur. The positioning of the engines and fuel within the aircraft must therefore be arranged so that the variations in _ the mass distribution caused as little departure as possible from the required elliptic pattern. It was clearly impossible at this stage to conclude this argument without knowing the proportions of the aircraft all-up weight due to engines and fuel respectively. Further information would be gleaned from subsequent discussion. Passing on to a consideration of the planform shape, the first problem again was that of drag. Supersonic flow theory was founded largely upon the concept of conical flow, the main hypothesis of which was that various quantities defining the flow remamed constant along straight radial lines emanating from the apex of the Mach cone, outside of which the free stream was undisturbed when quasi-subsomc condi tions obtained. This concept led one inevitably to the picture ot an aircraft shape developed from these conical lines. The low aspect ratio delta or dart was a typical example. An isosceles triangle as the plan with a single triangular fin whose base was equal in length to, and coincided with, the perpendicular bisector of the trailing edge, fulfilled this idea admirably. There was a wonderful economy in the straight line which made very little demand on the airflow, and it was rather extraordinary that, being perhaps nature's simplest gift to the geometer, it found so little use in current design. Perhaps its very simplicity offended the aesthetic taste. The delta configuration had several features to commend it, and also one serious drawback. For aspect ratios below 2, the delta had the large mean chord as required by the flutter problem; low frontal area and high radius or curvature for small profile drag; a useful wing thickness, even for the low thickness ratio, so that mass could be dis posed in the wings; high values for both flexural and torsional still nesses of the wings, obtainable with small skin gauge due to this thickness; and finally, a low moment of inertia and aerodynamic damp ing in roll, which implied smaller aileron loads than on a more conventional aircraft. The load density or aerodynamic pressure was smaller, with a consequent saving in stiffness required bv wing panels to normally applied loads. Against this, the most important disadvantage was the weight/span ratio mentioned in the drag discussion. Whereas the span might be small, the spanwise mass load-density distribution was high by virtue of the large mean chord. It could be shown that end-plates on the wings produced an effective increase in aspect ratio due to the reduction in spanwise circulation of the airflow and, for the purpose of induced drag, aspect ratio was effectively synonymous with the square of the span. Thus, by employing end-plates, the weight/span ratio could be decreased to a suitable value. It remained to decide what range of aspect ratios should be con sidered for this planform. It could be shown that the actual physical shift of the aerodynamic centres of a family of deltas possessing the same span in passing from subsonic to supersonic flow was independent of the aspect ratio or, what was equivalent, the root chord. Since this was the case, the lower the aspect ratio, the smaller would be the ratio of this shift to the root chord, with a consequent decrease in the relative difference between the respective trimming tail loads. It was clear that, from the tail load point of view, the smaller the aspect ratio the better, whereas from the induced drag point of view, the higher the better. Some compromise was obviously needed which took due account of the flight conditions previously mentioned, includ ing the reduction of profile drag with aspect ratio. From the aeroelastic point of view, a simple calculation on a swept wing had already shown a reduction of wing efficiency by about half, on a swept wing of aspect ratio 3 at relatively low supersonic speeds. The same sort of thing might be expected to occur with delta wings, and in particular the greater the span for a given skin thickness, the earlier and more marked would be the drop in efficiency. Thus, the range of aspect ratio which would be considered was between 1 and 1J. End-plates on the wing tips could increase this range by anything up to 20 per cent. In order that the cockpit could be included without any serious interference with the root profile, it was necessary that a depth of between 3 and 4ft could be obtained. A thickness ratio of between 4 and 5 per cent was desirable, so that for a span of 20ft, the root chord must be between 40 and 50ft. This concluded the planform discussion. With regard to the method of control, it was clear that, since the aerodynamic centre on deltas lay somewhat aft of 50 per cent root, the greatest moment arm for the tail was obtained by placing it at the nose of the aircraft. It was therefore suggested that, subject to satisfactory stability being possible, a canard delta, using an all-moving tail, could in part eliminate the undesirable effects of reversibility, although not preventing flutter. Some reduction in the flutter tendency might be achieved by employing an irreversible control system,' such as a screw jack, but it was possible that some alternative aerodynamic method could be designed which exhibited no flutter coupling while fulfilling the requirements of reversibility. Concerning aileron control, it seemed that rotating wing tips offered as good a chance as any of postponing reversal, while retaining a high rolling moment. Some difficulty might exist if end-plates were to be used, but these might act as an aerodynamic means of isolating the wing and control surface loads if placed over the junction of the two. These controls might be used differentially as ailerons, and in conjunction as elevons, at least in a secondary capacity. Finally, with regard to rudder control, there did not seem to be any great improvement possible on the existing flap system. This had in the past been the source of trouble in the flutter problem, and it was suggested that since directional control could be achieved by aileron plus elevator operation, the required effectiveness of the rudder could be reduced without much hazard. A three-view sketch of the configuration discussed in this section is shown in Fig. 3. FORTHCOMING EVENTS July 27- Aug. 1. Aero-Club de France: Parachuting Championships, Blois-Le- Breuil. Aug. 1. Opening of Jan Smuts Airport, Johannesburg. Aug. 1-3. S.M.A.E.: World Model Championships, Cranfield. Aug. 8-13. Round-Switzerland Air Tour. Aug. 22. A.T.A. Association: A.G M. and Summer Reunion, White Waltham. Aug. 22. Coventry Air Day and Siddeley Challenge Trophy Race. Aug. 29. Vintage Aeroplane Club: Rally, White Waltham. Aug.-*- 30. Aero-club d'ltalia: Pescara Rally. Sept. 3-10. Aero-Club de Cannes: Cannes Grand Prix. Sept. 6. S.M.A.E.: "Yorkshire Evening News" Model Flying Festival, Sherburn-in-Elmet. Sept. 7. S.B.A.C. Farnborough Display and Exhibition: Technicians' Day. Sept. 8-10. S.B.A.C. Farnborough Display and Exhibition: Private Invita tion Days. Sept.11-13. S.B.A.C. Farnborough Display and Exhibition: Public Days (Public premiere, Friday, 11th). 12. "At Home," R.N. Air Station Eglinton. 14 R Ae S. Wilbur Wright Memorial Lecture: "Structures," by Prof. N. J. Hoff, F.R.Ae.S., F.I.Ae.S. R.Ae.S. and I.Ae.S.: Anglo-American Aeronautical Conference. Battle of Britain Day. "At Home," R.N. Air Station Abbotsinch. 19. R.A.F. At-Home Day. 20. St. Albans Model Aero Club: All-Britain Model Aircraft Rally, Radlett. 20. Aero Club de Milan: Milan Grand Prix. 8. Start of England-Christchurch (N.Z.) Air Race. 17. Anniversary of the Wright Brothers' First Flight Sept. Sept. Sept.14-17 Seot. 15 Sept. 19 Sept. 19 Sept. 20 Sept. Oct. Dec.
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